Ceramic materials are becoming widely used in industry today. Ceramics because of their lower costs and weight are being substituted for steel and other metal-based materials in the construction of a variety of devices and machine parts. For example, ceramics are being used as replacements for steel and other metal materials in the manufacture of car bodies, airplane parts, turbine blades, flow control valves, engine parts, cutting tools, canning dies, and forging dies.
Similar components manufactured from alumina or zirconia are pressed from spray dried powders. The spray dry process agglomerates the high surface area powders with an organic that binds the particles together. The agglomerates are nearly spherical in shape and will flow uniformly filling dies of the desired shape. The powder is unaxially pressed at up to 30,000 psi, compacting the agglomerates. The binders allow the particle to rearrange during the process forming a uniform green body which after sintering yields high strength parts.
Sialon, an acronym for silicon aluminum oxynitride, is a particularly useful ceramic material which is generally prepared from a mixture of silicon nitride, alumina, aluminum nitride, and a sintering aid such as yttria or other rare earth oxide. As used herein, sialon powder includes any powder blend containing such a combination of components.
Aqueous processes have not previously been useful for preparing fine sialon powders because the nitride powder components react with water forming hydroxides on the particle surfaces and ammonia. During the drying step, whether cone blending or spray drying, very hard agglomerates are produced which do not compress during the green forming operation and which generate low density secondary agglomerates during subsequent sintering, reducing the strength of the final component.
Typically, fine sialon powder is produced by a non-aqueous process of milling in an alcohol such as isopropanol. The alcohol must then be evaporated using vacuum dryers, cone blenders, or spray dryers. Capital costs and operating costs are also increased over water milling due to the safety measures required when handling the organic solvents.
It is an object of the present invention to produce a sialon powder by an aqueous spray drying process. It is a further object to produce a sialon powder having improved strength as compared to prior such powders.